U.S. patent number 3,940,285 [Application Number 05/532,565] was granted by the patent office on 1976-02-24 for corrosion protection for a fuel cell coolant system.
This patent grant is currently assigned to United Technologies Corporation. Invention is credited to Richard C. Nickols, Jr., John C. Trocciola.
United States Patent |
3,940,285 |
Nickols, Jr. , et
al. |
February 24, 1976 |
Corrosion protection for a fuel cell coolant system
Abstract
The internal coolant system of a fuel cell power plant utilizes
a gas in the coolant fluid to inhibit the corrosion of those fuel
cell components that corrode due to shunt currents flowing through
the coolant fluid. In a preferred embodiment hydrogen gas is
used.
Inventors: |
Nickols, Jr.; Richard C.
(Glastonbury, CT), Trocciola; John C. (Glastonbury, CT) |
Assignee: |
United Technologies Corporation
(Hartford, CT)
|
Family
ID: |
24122302 |
Appl.
No.: |
05/532,565 |
Filed: |
December 13, 1974 |
Current U.S.
Class: |
429/434; 429/456;
429/524 |
Current CPC
Class: |
H01M
8/04029 (20130101); Y02E 60/50 (20130101) |
Current International
Class: |
H01M
8/04 (20060101); H01M 008/04 () |
Field of
Search: |
;136/86R,86B,86E |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mack; John H.
Assistant Examiner: Feeley; H. A.
Attorney, Agent or Firm: Revis; Stephen E.
Claims
Having thus described typical embodiments of our invention, that
which we claim as new and desire to secure by Letters Patent of the
United States is:
1. The combination of:
a fuel cell stack having a positive end and a negative end and
comprising a plurality of cells connected electrically in series,
said stack including components defining coolant channels for
carrying a coolant fluid into heat exchange relationship with said
cells and a coolant manifold passage extending from the positive to
the negative end of said stack for distributing a coolant fluid to
said channels, said components comprising an oxidizable material
and being in electrical communication with said fuel cells and
connecting them in series electrically; and
an ionically conductive coolant fluid within said manifold passage
providing a path for shunt current from the positive to the
negative end of said stack, said fluid including a gas mixed
therein which oxidizes at the positive end of said stack at an
electrochemical potential below which said material of said
components at the positive end of said stack are oxidized for the
level of shunt current through said fluid.
2. The combination according to claim 1 wherein said components
include a separator plate between adjacent fuel cells and a
separator plate at each end of said stack adjacent the end cells of
said stack.
3. The combination according to claim 1 wherein said gas includes
hydrogen.
4. The combination according to claim 2 wherein said plates include
carbon and said gas includes hydrogen.
5. The combination according to claim 3 wherein said coolant fluid
comprises water.
6. The combination according to claim 4 wherein said coolant fluid
comprises water.
7. The combination according to claim 2 including means for
introducing said gas into said coolant fluid.
8. The combination according to claim 7 wherein said gas includes
hydrogen.
9. The combination according to claim 1 wherein said components at
the positive end of said stack include an oxidation catalyst in
electrical communication therewith and in contact with said coolant
fluid to promote the oxidation of said gas.
10. The combination according to claim 9 wherein said components
include carbon, said gas includes hydrogen, said catalyst includes
platinum and said coolant fluid includes water.
11. The combination according to claim 1 wherein said components at
the positive end of said stack comprise platinized carbon, said gas
includes hydrogen, and said coolant fluid includes water.
12. In combination:
a fuel cell stack having a positive end and a negative end and
comprising a plurality of fuel cells connected electrically in
series and including an ionically conductive coolant fluid for
removing heat from said stack said fluid providing a path for shunt
currents from the positive to the negative end of said stack;
and
means for introducing an oxidizable gas into said coolant fluid,
said stack including oxidizable material in contact with said
coolant fluid at the positive end of said stack, said oxidizable
gas adapted to oxidize at the positive end of said stack at an
electrochemical potential below which said oxidizable material of
said stack at the positive end thereof is oxidized for the level of
shunt current through said fluid.
13. The combination according to claim 12 wherein said coolant
fluid includes water, said gas includes hydrogen and said material
includes carbon.
14. The combination according to claim 13 wherein said material at
the positive end of said stack includes platinized carbon.
15. In a process of operating a fuel cell stack having a positive
end and a negative end and comprising a plurality of fuel cells
connected electrically in series, said stack including an ionically
conductive coolant fluid for removing heat from said stack said
fluid providing a shunt from the positive to the negative end of
said stack wherein corrosion of fuel cell components at the
positive end of the stack and in contact with said coolant fluid
may occur due to the shunt currents flowing through said coolant
fluid, the process of preventing the occurrence of said corrosion
including:
introducing an oxidizable gas into said coolant fluid wherein the
gas is oxidized at the positive end of the stack in place of said
corrosion.
16. The process according to claim 15 wherein said gas includes
hydrogen and said coolant fluid includes water.
17. The process according to claim 16 wherein said fuel cell
components at the positive end of the stack in contact with said
coolant fluid includes carbon.
18. The process for preventing corrosion according to claim 15
including the step of providing an oxidation catalyst in contact
with said coolant fluid at the positive end of the stack and in
electrical communication with said fuel cell components at the
positive end of the stack.
19. The process for preventing corrosion according to claim 18
wherein the step of providing a catalyst includes platinizing said
carbon components at the positive end of said stack.
20. Fuel cell apparatus for preventing corrosion due to shunt
currents including, in combination:
a fuel cell stack having a positive end and a negative end and
comprising a plurality of fuel cells each including an electrolyte
between a pair of electrodes, a separator plate sandwiched between
adjacent fuel cells, and a separator plate at each end of said
stack adjacent the end cells of said stack, said separator plates
being in electrical communication with said fuel cells and
connecting them in series electrically, a plurality of said
separator plates having coolant channels therein for carrying a
coolant fluid into heat exchange relationship with said fuel cells,
said stack including at least one coolant fluid manifold passage
extending from the positive end of said stack to the negative end
of said stack for distributing coolant fluid to said coolant
channels;
means for introducing an ionically conductive coolant fluid into
said manifold passage and in contact with said separator plates
wherein shunt currents flow through said fluid from the positive to
the negative end of said stack; and
means for introducing an oxidizable gas into said coolant fluid,
said plates comprising material in contact with said coolant fluid
and oxidizable at the positive end of the stack at a potential
higher than the potential at which said gas is oxidized for the
particular level of shunt current flowing through said coolant
fluid.
21. The combination according to claim 20 wherein said coolant
fluid includes water and said plates comprise carbon.
22. The combination according to claim 21 wherein said gas includes
hydrogen.
23. The combination according to claim 22 wherein said plates at
the positive end of said stack include a hydrogen oxidation
catalyst in contact with said coolant fluid.
24. The combination according to claim 23 wherein said plates at
the positive end of said stack are platinized.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to fuel cells, and more particularly, to
corrosion prevention in the coolant systems thereof.
2. Description of the Prior Art
In a fuel cell stack the individual fuel cells are separated by
plates made of conductive material such as carbon. The plates
separate the reactant gases in adjacent fuel cells from each other
and are also usually electrically connected in series to each other
to carry electricity between the cells. A load is connected across
the stack to complete the circuit. During operation electrons flow
from the negative end of the stack to the positive end of the stack
through the load. There is generally a large potential drop from
one end of the stack to the other made up of smaller potential
drops between adjacent cells. Heat generated by the fuel cell stack
is often removed by flowing water or other fluids through channels
in the separator plates between the cells. These fluids are often
ionically conductive. Generally the water is manifolded to pass
through separator plates in parallel where it is collected in a
manifold at the other side of the cells; the heat of the cell may
change the water to steam which may be used in various components
of a fuel cell system, or the heat adsorbed by the water may simply
be radiated out to the atmosphere and the water recirculated
through the stack. Because of the potential difference between the
ends of the fuel cell stack and due to the manifolding of the water
from one end of the stack to the other, and because the water is in
contact with the current conducting and electrically connected
separator plates, shunt currents flow through the water. These
shunt currents cause the carbon plates nearest the positive end of
the stack to corrode with time which can be a serious problem in
fuel cells which must operate continuously for many thousands of
hours.
One solution, called edge cooling, involves flowing the coolant at
the edges of the cells only (i.e., no flow between cells). The
coolant is electrically insulated from the cells and thus no shunt
currents are present. This technique often results in an
unacceptable temperature distribution across the stack. Another
common solution is to use a dielectric coolant which cannot carry
current. However, a dielectric is not as good a coolant as water
and it may also be more expensive than water.
SUMMARY OF THE INVENTION
A primary object of the present invention is to inhibit the
corrosion that takes place in the coolant system of a fuel cell
stack due to shunt currents passing through the coolant fluid.
According to the present invention, an oxidizable gas is provided
in an ionically conductive coolant fluid of the fuel cell stack to
prevent corrosion of carbon and oxidizable metal components in
contact with the coolant caused by shunt currents flowing
therethrough. Hydrogen is a preferred gas because it is oxidized at
a potential lower than the potential at which carbon and metals are
oxidized at the level of shunt currents often encountered in a
stack. When hydrogen is added to the coolant the hydrogen is
oxidized rather than the oxidizable material of the coolant system
component. The hydrogen is readily available for this purpose since
it or a hydrogen containing gas is usually used as fuel for the
stack.
In a preferred embodiment a hydrogen oxidation catalyst (such as
platinum) is provided in electrical contact with the corrodible
component to decrease the electrochemical polarization at a given
level of shunt currents; another way to put it is that the catalyst
increases the tolerable level of shunt currents by increasing the
limiting shunt current level for hydrogen oxidation.
The subject matter of this application is related to the subject
matter of commonly owned United States patent application titled
"Corrosion Protection for a Fuel Cell Coolant System" by M. Katz,
S. Smith, and D. Reitsma filed on even date herewith.
The foregoing and other objects, features, and advantages of the
present invention will become more apparent in the light of the
following detailed description of preferred embodiments thereof as
illustrated in the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is illustrative of a fuel cell stack which utilizes the
present invention.
FIGS. 2-5 are graphs illustrative of various electrochemical
reactions which may occur in the coolant system of a fuel cell
stack of the type illustrated in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is an illustrative representation of a typical fuel cell
stack 10 in which the present invention may be used. The stack 10
is comprised of a plurality of fuel cells 12 each including an
anode electrode 14 and cathode electrode 16. Trapped between these
electrodes is an electrolyte containing matrix 18 which holds a
liquid electrolyte in contact with the surfaces of the electrodes
14, 16. (This invention is also useful for other types of fuel
cells wherein the electrolyte is a free liquid, rather than
retained in a matrix.) The individual cells 12 are separated by
separator plates 20, the first and last plates 20 of the stack
generally being referred to as end plates rather than separator
plates because they do not actually separate adjacent cells. Each
of the plates 20 includes grooves 22 formed therein which define
fuel and oxidant gas spaces 24, 25 adjacent the nonelectrolyte
sides of the electrodes 14, 16, respectively. Means are also
provided, but are not shown, for introducing and removing the fuel
and oxidant from the gas spaces 24, 25 in accordance with well
known techniques. The plates 20 are made from an electrically
conductive material such as carbon (The term "carbon" is intended
to encompass "graphite" within its meaning.) or a metal and are
connected electrically in series with each other and with a load
26. Electrically insulating end seals 28 help retain the
electrolyte within the matrix 18 and also prevent the electrodes of
the individual cells 12 from shorting out.
The plates 20 also include coolant channels 30 in heat exchange
relationship with the cells 12. The channels 30 are fed from a
coolant fluid manifold passage 32 and feed into a manifold passage
34 on the other side of the stack 10. The manifold passages 32, 34
may simply be a plurality of interconnected cylindrical passageways
through the plates 20 and end seals 28. An ionically conductive
liquid coolant is fed into the manifold passage 32 through inlet
means 36. The coolant thereupon passes through the channels 30 of
the separator plates 20 picking up heat generated by the cells 12
and exits from the stack through outlet 38. Although in this
embodiment there are coolant channels 30 between every pair of
cells 12, it may be sufficient in certain instances to pass coolant
fluid through only every other plate 20 or through any suitable
number of plates, but generally at regular intervals, as long as
the temperature in one cell does not exceed its maximum operating
temperature and as long as the temperature distributions within and
between cells is tolerable.
Water is a very desirable and common coolant which is often used
alone or mixed with an additive such as glycol to alter the boiling
and freezing point. Manifolding the water, as in the embodiment of
FIG. 1, results in an unbroken path of liquid through the manifold
passage 32 from the positive end 40 of the stack to the negative
end 42. Since the separator plates between adjacent cells are
electrically conductive and connected in series through the cells
12, a large electrical potential may exist across the coolant fluid
in the manifolds 32, 34 from the positive to negative end of the
stack. This potential exists whether or not the circuit connecting
the cell stack to the load 26 is open or closed (although it is
somewhat lower when the circuit is closed). In order for shunt
currents to flow through the coolant there must be some kind of
electrochemical reaction at both the positive and negative ends of
the stack at the interface between the coolant and the fuel cell
coolant system components (which in this embodiment are the plates
20). In the case of an aqueous coolant hydrogen ions are produced
at the positive end 40 of the stack [see equation (1) below] and
flow through the water in the manifold passage to the negative end
42 of the stack where it is converted back to hydrogen gas. This
shunt current corresponds to a load or drain on the stack 10.
For a given stack voltage (which is dependent on the number of
cells in the stack) the magnitude of the shunt current through the
water depends on the electrical resistance of the coolant fluid and
the electrochemical reactions in the fluid at the positive and
negative ends of the stack. Most of the potential drop across the
coolant fluid is due to its electrical resistance. For example, in
a 300 volt stack the IR drop across the coolant may be about 299
volts; this would leave only about 1 volts for the electrochemical
reactions. The graphs of FIGS. 2-5 depict the electrochemical
potential-shunt current relationships of various reactions which
may occur between an aqueous coolant fluid and elements in the
fluid; or between the fluid and the coolant system component in
contact therewith.
Referring to FIG. 2, assume for the moment that the separator
plates 20 contain carbon and the coolant is ordinary water with no
additives and some ionic conductivity (which is the usual case). It
can be seen that at an electrochemical potential of about 0.95 volt
or above, the carbon in the components at the positive end 40 of
the stack begin to react with the water when even the slightest
positive shunt current exists according to the following
formula:
This represents carbon corrosion of fuel cell components and is
counter-balanced at the negative end 42 of the stack by the
electrolysis of water to hydrogen according to the following
reaction:
Notice in FIG. 2 that when the shunt current reaches a level of I'
there may also be oxygen evolution (the electrolysis of H.sub.2 O
to O.sub.2) at the positive end of the stack (as well as carbon
corrosion) according to the following reaction:
At this point the potential has exceeded about 1.2 volts, the
theoretical voltage for oxygen evolution. As heretofore discussed
it is an object of the present invention to prevent this corrosion
at the positive end of the stack.
Consider, now, what happens when hydrogen is added to the water in
accordance with the present invention. This is represented by the
graph of FIG. 3 which is the same as the graph of FIG. 2 except for
the presence of the hydrogen oxidation curve. It can be seen that
for currents below about I" hydrogen is oxidized at a potential
less than the potential required for the initiation of carbon
corrosion. Thus, as long as the shunt currents are less than I"
oxidation of hydrogen will take place at the positive end 40 of the
stack according to the following reaction:
and there will be no carbon corrosion or oxygen evolution. When
hydrogen oxidation occurs the corresponding reaction at the
negative end 42 of the stack is the electrolysis of water to
hydrogen (hydrogen evolution) according to the following
reaction:
The coolant used in the embodiment of FIG. 1 may, for example, be
ordinary water plus hydrogen. The water may be converted to steam
as it flows through the stack 12 and the steam from the stack may
be used in a steam reformer for processing fuel. The hydrogen in
the coolant is consumed at the positive end 40 of the stack 12 and
evolved at the negative end 42. Thus, the steam leaving the stack
by way of outlet 38 includes substantially the same amount of
hydrogen as introduced into the coolant. It has been found that the
hydrogen has no detrimental effect in the steam reformer. Since the
hydrogen is not recovered there must be a continuous feed of
hydrogen into the water. If a supply of pure hydrogen is available
it can be injected directly into the water. For example, in the
embodiment of FIG. 1 there is shown a hydrogen supply 44, the
hydrogen being fed directly into the coolant fluid and being
controlled by a valve 46. If the hydrogen is to be supplied from a
hydrogen containing gas, such as the fuel gas for the fuel cell,
pure hydrogen can be introduced into the water perhaps by diffusing
the hydrogen through a suitable hydrogen diffusion membrane which
prevents other components of the gas from entering the water. If
these other gas components are introduced into the water, some of
them might be harmful to the stack or to the steam reformer. If
these other components are not harmful a hydrogen diffusion
membrane may not be needed.
Although in this embodiment the coolant fluid is water and the
oxidizable gas is hydrogen, the coolant fluid may be any ionically
conductive fluid; the gas may be any gas which is not harmful to
the system and which is oxidized at a potential lower than the
potential at which the material of the coolant system corrodes at
the level of shunt currents encountered, such as carbon
monoxide.
It is also contemplated that this invention may be used in fuel
cell systems wherein the coolant is contained in a recirculating
loop which passes through the fuel cell stack. In that type of a
system the coolant may be any ionically conductive fluid such as
ordinary water or may be water mixed with an additive such as
glycol. The hydrogen (or other oxidizable gas) which is added to
the coolant is consumed at the positive end of the stack and is
evolved at the negative end of the stack and is thereupon
recirculated with the coolant from the outlet 38 to the inlet 36
and back to the positive end of the stack. Thus, the amount of
hydrogen in the coolant remains substantially constant. A
continuous supply of hydrogen to the coolant is therefore
unnecessary in this embodiment except to make up for incidental
losses due to leaks; this makeup hydrogen could be provided from
the hydrogen supply 44.
In the oxidation of hydrogen according to reaction (5) the carbon
of the coolant system components (i.e., plates 20) at the positive
end of the stack acts as a catalyst, although not a very good one.
However, it may be suitable for very small shunt currents. Reaction
(5) can be enhanced by adding any good hydrogen oxidation catalyst
to the carbon components at the positive end of the stack such as
by platinizing the carbon surfaces thereof. The graph of FIG. 4 is
indicative of the electrochemical reactions occurring at the
positive end 40 of the stack on a platinized carbon surface with
hydrogen present in the water coolant. The platinum catalyst
increases the rate of the electrochemical reaction at a given
potential so that a shunt current of I" may be present before
carbon corrosion begins. The catalyst need only be present at the
positive end of the stack, and may, for example, simply be a rod of
platinum extending into the coolant fluid and in electrical contact
with the carbon component. Other possible catalysts are any noble
metal or nickel, although this invention is not intended to be
limited thereto.
In general terms the reaction for metal corrosion at the positive
end of the stack is represented generally as follows:
and may occur if no hydrogen were present in the coolant and also
at shunt currents greater than I.sub.a even if there were hydrogen
in the coolant. Metal deposition (i.e., plating out) would be the
corresponding reaction at the negative end of the stack as
follows:
As well as preventing carbon corrosion, the addition of hydrogen to
the coolant may be useful in preventing the corrosion of metals in
general. For example, the addition of hydrogen can prevent the
corrosion of metals such as nickel or stainless steel which are
oxidizable at a potential higher than the hydrogen oxidation
potential for the level of shunt currents usually encountered. This
is shown in the graph of FIG. 5. A typical metal
oxidation-reduction curve (i.e., metal corrosion curve) is shown in
place of the carbon corrosion curve. This curve is representative
of equation (6) above. This graph is purely illustrative, and the
metal corrosion curve is not intended to represent any particular
metal but is to illustrate that the oxidation of hydrogen occurs at
a lower potential than the potential at which corrosion of some
metals is initiated such that shunt currents are tolerable up to a
value of I.sub.a. Platinizing of the metal surface would also be
advantageous in this situation. Thus, with hydrogen present in the
coolant according to the present invention and shunt currents less
than I.sub.a the reaction at the positive end of the stack is
oxidation of the hydrogen (instead of metal corrosion) and at the
negative end of the stack is electrolysis of water to hydrogen
(instead of metal plating out), as hereinabove set forth in
reactions (4) and (5), respectively.
Although the invention has been shown and described with respect to
preferred embodiments thereof, it should be understood by those
skilled in the art that various changes and omissions in the form
and detail thereof may be made therein without departing from the
spirit and the scope of the invention.
* * * * *